In general, a resistive element to be used in a semiconductor integrated circuit is fabricated by a process including the steps of ion implantation of impurity into a semiconductor wafer; and a thermal treatment, for example, annealing, to activate it. For instance, boron is implanted into a semiconductor Si wafer through the screen oxide layer. Next, annealing process is carried out to the wafer to form a P-type diffused resistor.
However, according to a conventional method, a variety of electric charge is trapped between a screen oxide layer and a semiconductor wafer, and therefore, a surface potential of the semiconductor wafer may be changed. As a result, characteristics of a diffused resistor formed on the semiconductor wafer are fluctuated. Such a fluctuation is caused by electric charge trapped between a screen oxide layer and a semiconductor wafer. This kind of problem is remarkable for a high-resistivity element.
Since electric charge is trapped on a side of an oxide layer, such electric charge can be removed by removing the oxide layer with hydrofluoric acid. However, another electric charge is generated in following thermal treatments. Conventional technology relating to a density of electric charge is described in a reference, J. Electrochem. Soc: SOLID STATE SCIENCE, March 1967, pp 266-273, and J. Electrochem. Soc: SOLID STATE SCIENCE, September 1971, pp 1463-1468.